High specific electromechanical characteristics (power output, torque, electromotive force per unit mass of the machine) are very important for electric machines having permanent magnets.
This objective can be attained by increasing the intensity of the magnetic flux produced by the multipolar rotor of an electric machine by making the rotor magnets from anisotropic magnetically hard materials having high magnetic characteristics, better utilization of the rotor volume for disposing the magnets and optimum orientation of their lines of force.
Known in the art is a multipolar cylindrical rotor of an electric machine, comprising prism-shaped permanent magnets disposed uniformly around a circle and magnetized along their axes of symmetry, each magnet being provided with a pole piece made of magnetically soft material. For complete filling of the rotor volume with magnetically hard material, prismatic magnets have tapered side faces by which the adjacent magnets join one another.
The permanent magnets are secured on a bushing made of a magnetically soft material; arranged between these permanent magnets are inserts of a nonmagnetic material.
The above rotor does not allow its volume to be completely filled with magnetically hard material, which is spaced from the rotor surface due to the presence of pole pieces, and this does not allow the magnetic flux to be increased significally. In addition, the presence of pole pieces reduces the magnetic flux due to appearance of a leakage flux between the adjacent pole pieces, which increases eddy-current and remagnetization losses and also increases the electric machine inductance.
The magnetic flux of such a magnet is produced by the magnetic charges distributed over the poles, which are arc-shaped in cross section; the length of this arc is much lower than the pole division of the rotor, as well as the magnetic charges distributed on the surface of the magnetically soft bushing and producing a magnetic flux of the opposite direction with respect to the magnetic flux of the charges distributed over the poles. These two factors: a shortened pole arc and the presence of an oppositely directed magnetic flux in the magnetically soft bushing reduce the magnetic flux per pole. Furthermore, the shortened arc results in quick drop of induction in the gap beyond this arc, reducing the mechanical torque produced by the motor.
Known in the art is a cylindrical rotor with permanent magnets for electric machines, comprising pole sections made of a magnetically soft material disposed uniformly along a circle, and interpole sections made of a magnetically hard material magnetized normal to the respective planes of symmetry of the adjacent pole sections.
The concentration of the magnetic flux in the gap between the above described rotor and stator of the electric machine is obtained due to the presence of sections of magnetically soft materials in this gap.
The above rotor of an electric machine, having definite number of poles and small working gaps, increases the magnetic flux but, at the same time, increases the leakage flux due to the presence of sections of magnetically soft material, as well as the inductance of the electric machine with such a rotor thus hindering the electric drive control.
Also known in the art is a rotor of an electric machine, made in the form of a circular cylinder and comprising an even number of pole magnets (at least four magnets) disposed uniformly around a circle; some poles of these magnets face the center of the circle while the other poles face its periphery and alternate; the interpole magnets adjoining the pole magnets have side surfaces turned to the circle periphery.
The pole magnets are equipped with pole pieces made of a magnetically soft material, which in combination with the interpole magnets form a cylindrical surface of the rotor. The pole magnets are secured on a bushing made of a magnetically soft material and are magnetized in the radial direction, while the interpole magnets are mounted with a gap relative to the bushing and are magnetized along the arcs of circles. In this rotor the presence of pole pieces orienting the lines of force of the magnetic field normal to the working surface of the rotor results in that the pole magnets do not face the side surface of the rotor, while the leakage fluxes are not fully eliminated so that the magnetic flux is reduced. Furthermore, the addy-current and remagnetization losses increase, as well as the electric machine inductance.
In this case the radial direction of magnetization of the pole magnets and the arc-shaped direction of magnetization of the interpole magnets are difficult to provide technologically, while attainment of such a direction of magnetization would impair the magnetic characteristics of the magnetically hard materials. If radial magnetization is achieved, magnetic charges appear inside the pole magnets, which reduce the excitation magnetic flux.
The presence of a bushing made of a magnetically soft material does not allow the rotor volume to be used entirely for mounting the magnets, reduces the rotor resistance to demagnetization, causes eddy-current and remagnetization losses and increases the electric machine inductance.